1. Field of the Invention
The present invention relates to an ink jet recording apparatus and method and, more particularly, to a recording apparauts and method, which heat a liquid using electrothermal energy converting elements, and eject liquid droplets.
2 . Related Background Art
In recent years, office automation ("OA") equipments such as computers, wordprocessors, copying machines, and the like have become popular, and a large number of recording apparatuses for such equipment have been developed. An ink jet recording apparatus allows easier high-definition recording than other recording methods, and can realize low-noise, low-cost recording at high speed.
An ink jet recording apparatus is designed to have various recording modes in addition to a simple one-scan recording mode so as to solve a problem in fixing characteristics of an ink on recording media such as a paper sheet, an overhead projection ("OHP") film, and the like, and to prevent, e.g., density nonuniformity inherent to recording heads.
Of these recording modes, a multi-pass print mode for performing recording by scanning a single recording head a plurality of number of times for a single area on a recording medium is popularly used for various purposes. In particular, when color recording is performed by a plurality of recording heads using different colors of inks, a thin multi-pass print mode is executed. In this mode, in order to prevent ink overflow and image blur on the recording medium, instead of printing all the recording data at one time, a single scan pattern is thinned out in a checkered pattern, and the checker pattern is recorded a plurality of number of times, thereby finishing an image corresponding to all the recording data. Even in a monochrome recording mode, the following print method as a modification of the thin print multi-pass print mode is executed. For example, in a recording head consisting of a plurality of nozzles, density nonuniformity inherent to the recording head due to a difference in ejection characteristics among the nozzles is often observed. In order to eliminate such nonuniformity, half of all the recording data is printed by a first nozzle group, and the remaining half is printed by a second nozzle group.
In the thin multi-pass print mode, the density of a recorded image tends to become lower than an image obtained when recording is performed at a time without using a thinning pattern (one-pass print mode). The degree of this phenomenon varies depending on the types of recording media. In general, this phenomenon is less likely to occur on coated paper coated with an ink reception layer, and is often observed on non-coated paper such as plain paper copier ("PPC") paper having no special coat. Although the details of the mechanism of this phenomenon are unknown, this phenomenon has a strong correlation with the ink absorption speed of ink reception characteristics, and readily occurs on paper which has poor ink fixing characteristics. Since this phenomenon is associated with ink absorption in a recording medium such as paper, it also has a correlation with the ink composition. Although paper having poor fixing characteristics requires the thin multi-pass print mode, a decrease in density easily occurs on such paper due to the thin multi-pass print mode.
In an ink jet recording method, ink droplets are formed by various methods, and recording is realized by depositing ink droplets onto a recording medium such as paper.
Of recording apparatuses adopting a recording method of this type, as an apparatus having a structure suitable for a high-density multi-orifice recording head, an ink jet recording apparatus of a type utilizing heat as energy for forming ink droplets is known.
The ink jet recording apparatus, which utilizes heat as ink droplet ejection energy, normally comprises a recording head having ink droplet forming means for heating an ink to displace the ink by causing an abrupt increase in the volume of the ink, and ejecting the ink from orifices of a nozzle section, thereby forming ink droplets, i.e., electrothermal energy converting elements, which generate heat upon application of an electrical signal, and which can heat the ink.
In the ink jet recording apparatus, the ink jet characteristics, especially, the ink droplet size, are influenced by the temperature of the recording head, and the print density changes depending on the temperature. Thus, temperature control for, e.g., maintaining a constant temperature of the recording head is performed.
FIG. 22 shows an ink jet printer head for heating an ink by a heater to generate bubbles, and ejecting ink droplets in response to the bubbles, and in particular, shows details of an ejection element 58 as the principal part of the head.
Heaters H.sub.1 65 and H.sub.2 66 used for heating the head and keeping the head temperature are formed on an Si substrate 61 from the same material as that of ejection heaters 63. Energization of the heaters H.sub.1 65 and H.sub.2 66 is ON-OFF-controlled according to head temperature information from a temperature detection means (thermistor 59) mounted on a base plate 53, thereby controlling the head temperature. Orifices 62 for ejecting ink communicate with corresponding nozzles 64. The nozzles 64 are supplied with ink from an ink tank (not shown) through an ink chamber 68. A filter 69 is arranged on the ink chamber 68. The heaters 63, 65, and 66 are connected to an electrical circuit board 54 via an Al wiring pattern 67 and bonding wires 70.
However, since the ink jet recording head for ejecting ink droplets by utilizing heat energy generates heat by itself upon recording, the ink temperature in each nozzle where the ejection heater is arranged is higher than a temperature detected by the temperature detection means during recording, and this temperature difference varies depending on the recording pattern and the record density. For this reason, when data having a high record density such as an image pattern is to be recorded, the ink temperature in the nozzles is increased, and the print density is increased accordingly. On the other hand, when data having a relatively low record density such as a character pattern is to be recorded, the print density is lowered. It is therefore difficult to always obtain a uniform print density.
When an image pattern is recorded, a multi-pass print operation for performing one-line recording for a plural number of times of carriage scan operations is often performed so as to improve image quality (color boundary blur, color misregistration, and the like). In this case, the print density may vary due to a difference in head temperature rise caused by different print densities in respective scan operations. When the print density in each scan operation varies, the ink penetration state onto a recording medium varies. In general, as compared to a case wherein the print density is increased by a one-pass print method, when an ink is printed a plurality of number of times at a low print density, the spread of each ink droplet on a recording medium is decreased, and the apparent print density is undesirably lowered.
The demand for an ink jet recording apparatus capable of performing color recording is increasing. Such a recording apparatus uses a recording head having an array of a plurality of recording elements obtained by integrating a plurality of ink ejection orifices and nozzles. Some recording apparatuses have a plurality of recording heads for respectively ejecting cyan, magenta, yellow, and black inks for color recording. The ink ejection orifices of such a recording head eject ink in almost an equal quantity per pixel.
However, the ink ejection quantity is considerably influenced by the temperature of the recording head. More specifically, when the temperature of the recording head is high, the ejection quantity is increased; when the temperature of the recording head is low, the ejection quantity is decreased. Such a difference in ejection quantity largely influences the density of a printed image. In order to stabilize the ejection quantity independently of the environmental condition of the recording head, temperature control must be performed for each recording head.
For this purpose, a temperature control heater is arranged near or in the recording head in addition to ink ejection heaters, and the temperature of the recording head is detected by a thermistor. The detected temperature is fed back to a control mechanism to stabilize the ink ejection quantity, thereby obtaining an image free from print density differences and which is independent of the environmental temperature of the recording head. In this case, temperature control is uniformly performed for recording heads. On the other hand, when color recording is performed using ink jet recording heads, the following method is adopted. That is, coated paper having good ink absorbency is used as a recording medium, ink droplets each having a relatively small drop size are ejected onto the coated paper, and inks are absorbed in the coated paper.
When the above-mentioned coated paper is used as the recording medium, a sufficient print density and fixing characteristics can be assured. However, a demand has arisen for a print operation using low-cost regular paper as a recording medium with the recent advent of low-cost information and communication equipment. When a recording apparatus is designed exclusively for coated paper, a user must use expensive recording media, and cannot desirably select other recording media.
Since regular paper has no special treatment for absorption of an ink as a liquid, it cannot easily obtain a sufficient density as compared to coated paper manufactured in consideration of ink absorbency. In particular, the density of a black ink used for characters, ruled lines, and the like is important in an image. Therefore, to obtain a sufficient density is an important subject for the print operation on regular paper.
When temperature control upon ejection is uniformly performed for a plurality of recording heads like in the above-mentioned conventional method to perform a print operation on recording media such as regular paper, OHP paper, and the like having inferior ink absorbency to that of coated paper, since ink droplets to be ejected are set to have a small drop size, as described above, the density of black characters and black lines is low. In addition, since inks are implanted in too great quantities on R (red), G (green), and B (blue) portions obtained by mixing inks ejected from the cyan, magenta, and yellow recording heads so as to obtain a color image, the inks are considerably blurred, thus deteriorating print quality.
Furthermore, since the recording heads suffer from variations in ejection quantity, the print density varies in units of recording heads. As a result, especially in a color recording mode, different color tones are obtained in units of color ink jet recording apparatuses.